Mystery of Apparent Wind
Sails operate in apparent wind, which is a combination of your boat’s speed and the true wind (the wind you experience when your boat is not moving).
To sail efficiently, sails must be trimmed to the apparent wind.
Knowing this gives racing
sailors an edge over their competition, and it enables cruising sailors to reach their destinations much faster.
Let’s break the mystery of apparent wind into four easy pieces.
Picture yourself standing outdoors with the wind blowing against the left side of your face. Begin walking, and increase your pace until you reach jogging speed. Then slow down gradually until you’re standing still.
What did you notice about the wind?
When standing still, you felt the true wind against the left side of your face.
As you increased your speed to a jog, the wind “appeared” to move toward the front of your face.
Of course, the wind never
really moved; rather, your own forward motion, or speed, caused this apparent wind.
Remember these two facts about apparent wind.
If boatspeed increases, apparent wind moves toward the bow (or the front of your face).
If boatspeed decreases, apparent wind moves away from the bow (or away from the front of your face).
Boat course changes.
If two vehicles traveling at 25 mph were to collide, the drivers would feel an impact of 50 mph.
Similarly, when we are sailing close-hauled (as close to the wind as possible), our boat speed and the true wind speed are additive, and the apparent wind feels stronger than the true wind.
On the other hand, if both cars were traveling in the same direction and one hit the other, neither driver would feel much impact because the diff erence in their speeds would be negligible.
Similarly when we are running downwind (with the true wind behind the boat), our boatspeed partially counteracts the true wind speed, and the apparent wind is lighter than the true wind.
Remember these two facts about course changes and apparent wind:
If you change course toward the
true wind, apparent wind speed increases.
If you change course away from
the true wind, apparent wind speed decreases.
When you are closed-hauled, the apparent wind is stronger than the true wind. When you are running, the apparent wind is not as strong as the true wind.
True wind speed changes.
In sailing, we deal with three phases of true wind:
the steady wind phase, the gust wind phase, and the lull wind phase.
Gust and lull wind phases operate like the accelerator pedal in your car. If you need to pass another vehicle, you accelerate for a short while.
If you need to slow down to allow a vehicle to pass you, you decelerate for a short while.
Lift s and headers can occur not just when wind direction changes,
but when velocity changes as well.
A wind gust accelerates above the baseline steady wind for a short period of time.
Sailors call a gust from the same direction as the baseline breeze a velocity lift because the apparent wind moves away from the bow.
If you’re sailing close-hauled, you can point your boat closer to your windward destination during a
gust from the baseline wind direction.
As your boat speeds up in response to the stronger wind, however, the apparent wind will move forward again—though perhaps not enough to force you all the way down to your previous baseline close-hauled course.
A lull decelerates below the steady wind for a short period of time.
Sailors call a lull from the baseline wind direction a velocity header because the apparent wind moves toward the bow.
When sailing close-hauled, you will have to respond by falling off the wind somewhat, which points you farther from the windward destination you are trying to reach.
You should always tack on a header to keep moving toward your destination, but a velocity header and a header resulting from a change of true wind direction are different animals.
A velocity header will cease to be a header once your boatspeed drops in response to the lull.
As your boatspeed falls—and assuming the true wind direction remains steady—the apparent wind will move back aft to its previous position.
Thus, tacking will gain you nothing unless sailing the other tack will get you into a stronger breeze.
Look at the wind on the water ahead and to windward. Darker streaks denote areas of stronger breeze.
Look at other sailboats too—do they seem to be heeling more and moving faster to windward or ahead?
Weigh the evidence and decide whether the wind is more likely to fill back in if you hang with your current tack or switch tacks.
Remember these two facts about gusts and lulls:
If true wind speed increases, the apparent wind moves away from the bow (a velocity lift ).
If true wind speed decreases, the apparent wind moves toward the bow (a velocity header).
True wind direction changes.
A change in the true wind direction has an equally signifi cant though more subtle effect on off wind sailing.
One thing that makes sailing downwind difficult is that a small change in true wind direction results in larger changes in apparent wind direction.
Keep in mind that sailing vessels sail to the apparent wind.
According to Steve Colgate, founder of the Off shore Sailing School, a 16-degree change in the true wind makes a 28-degree change in the apparent wind when you’re sailing downwind.
That’s one reason boats sometimes roll from side to side when running before the wind.
The person at the helm might be oversteering (moving the wheel or tiller too much) to try to keep the boat on a straight course.
Oversteering in combination with a big apparent wind change could result in an accidental jibe, in which the boom swings across the boat with great force, possibly damaging the rig or causing serious injury.
In gusty downwind sailing conditions, keep the wind over the stern quarter rather than directly over the transom, and rig a boom preventer or lower the mainsail and sail under headsail alone.
Remember this when sailing downwind:
When well off the wind, a small change in true wind direction causes a large change in apparent wind direction.